Alberta was the province with the highest proportion of NOX emissions, with 35% (719 kt) of national emissions (2061 kt). The oil and gas industry emitted the largest amount of NOX for this province. Ontario contributed the second-largest proportion of NOX, with 16% (339 kt) of national emissions. British Columbia ranked third, with 13% (275 kt) of national emissions. In both Ontario and British Columbia, transportation (road, rail, air and marine) was the most important source of NOX emissions. The oil and gas industry was also an important source of NOX emissions in British Columbia. Off-road vehicles were the second most important source in Ontario. Ontario experienced the largest reduction in emissions between 1990 and 2013 mainly due to emission reductions from transportation and electricity power-generating plants.